1. Field of the Invention
The present invention relates to a method for manufacturing a piezoelectric device that uses a thin film made of a piezoelectric single crystal material.
2. Description of the Related Art
Many kinds of piezoelectric devices that utilize a piezoelectric thin film, i.e., thin-film piezoelectric devices, are now under development. There are many ways to produce a piezoelectric thin film for the fabrication of thin-film piezoelectric devices. For example, Japanese Unexamined Patent Application Publication No. 2002-534886 proposes a method for manufacturing a piezoelectric device in which a piezoelectric thin film is formed by isolation from a piezoelectric substrate at an ion-implanted layer as the isolation plane.
The following describes this manufacturing method of a piezoelectric device, which includes forming a piezoelectric thin film by isolation from a piezoelectric substrate at an ion-implanted layer as the isolation plane, with reference to FIGS. 1A-1D.
FIGS. 1A-1D are schematic cross-sectional diagrams illustrating the manufacturing steps of a piezoelectric device according to Japanese Unexamined Patent Application Publication No. 2002-534886. The first step is to implant hydrogen ions into a piezoelectric substrate 5 from the front 7 side in a way that an ion-implanted layer 6 is formed at a predetermined depth d in the piezoelectric substrate 5 as illustrated in FIG. 1A. A coupling material 8 is then deposited on the front 7 of the piezoelectric substrate 5 by sputtering as illustrated in FIG. 1B. The next step is to join the piezoelectric substrate 5 and a supporting substrate 9 as illustrated in FIG. 1C. Finally, the joined body of the piezoelectric substrate 5 and the supporting substrate 9 is heated to initiate isolation at the ion-implanted layer 6 as the isolation plane. As a result, a piezoelectric thin film 5′ is formed on the supporting substrate 9 as illustrated in FIG. 1D.
In the manufacturing method according to Japanese Unexamined Patent Application Publication No. 2002-534886, however, ion implantation (see FIG. 1A) causes the piezoelectric substrate 5 to be warped with the ion-implanted layer 6 side of the piezoelectric substrate 5 curved outwards as illustrated in FIG. 2. This type of warpage occurs because the implanted ionized element causes the crystal lattice spacing of the piezoelectric material to extend in the ion-implanted portion of the piezoelectric substrate 5.
When the piezoelectric substrate 5 is in such a warped shape, bonding it to a supporting substrate 9 (see FIG. 1C) results in the piezoelectric substrate 5 adhering to the supporting substrate 9 with the crystal lattice spacing longer than that before ion implantation. The crystal lattice spacing in the piezoelectric thin film 5′, which is left after bonding to the supporting substrate 9 and isolation at the ion-implanted layer, is therefore longer than that before ion implantation. It is, however, generally known that a piezoelectric thin film 5′ having an extended crystal lattice spacing as above has lower piezoelectricity than one with no expansion of the crystal lattice spacing.
This means that piezoelectric thin film devices produced by the manufacturing method according to Japanese Unexamined Patent Application Publication No. 2002-534886 are disadvantageous because of degraded piezoelectricity. This is a fatal problem for the device characteristics of high-frequency filters in particular because in devices of this type, an extended crystal lattice spacing is a big factor for the degradation of piezoelectricity.
Heat treatment for the recovery of piezoelectricity expels the ions of the implanted element existing between the crystals from the piezoelectric thin film 5′ by degasification and is expected to make the crystal lattice spacing of the piezoelectric thin film 5′ return to that before ion implantation. However, the piezoelectric thin film 5′ is bonded to the supporting substrate 9; it is confined by the supporting substrate 9 and unable to regain its crystal lattice spacing before ion implantation. In other words, the piezoelectric thin film 5′ formed on the supporting substrate 9 is under shear stress. Piezoelectric thin film devices produced by the manufacturing method according to Japanese Unexamined Patent Application Publication No. 2002-534886 therefore has another disadvantage in that the piezoelectric thin film 5′ is likely to be damaged.